The present invention relates to a video camera that generates a video signal by mixed-line-pair readout from a solid-state image sensor with a complementary color filter.
Video cameras with solid-state image sensors can take both moving and still pictures of rapidly moving subjects. A still picture of a rapidly moving object can be obtained by use of a high-speed mechanical shutter, or by illuminating the subject for a short interval with a strobe lamp. Since both of these methods require bright illumination of the subject, a system using only strobe illumination is less expensive than a system that also uses a mechanical shutter.
Owing to their small size and high reliability, charge-coupled devices (CCDs) have come into common use as solid-state imager sensors. The CCD image sensors used in video cameras are area sensors comprising a matrix of sensor elements or pixels. The image signal is acquired in the form of electrical charge by the sensor elements, read into a plurality of vertical transfer registers, then shifted into a horizontal transfer register for output as a video signal.
A complementary color filter provides individual filtering of the light incident on each pixel, in a pattern of complementary colors that enables a color video signal to be derived from the output of a single CCD image sensor. CCD image sensors with complementary color filters are generally used in video cameras, such as television cameras, that employ interlaced scanning. Mixed-line-pair readout is a useful method of creating an interlaced color video signal.
In a CCD image sensor employing mixed-line-pair readout, the number of stages in each vertical transfer register is only half the number of horizontal pixel lines, and the vertical transfer registers receive the image signal one field at a time, rather than one frame at a time. When signal charges are read from the sensor elements into the vertical transfer registers, the signal charges stored in two vertically adjacent pixels are read into the same register stage and combined. The combined charges are transferred into the horizontal transfer register one line at a time, for serial output as a video signal. Each line read from the horizontal transfer register thus represents the sums or averages of two adjacent horizontal lines of sensor pixels.
To produce the necessary one-line offset between even fields and odd fields, a given horizontal line of pixels is combined with the horizontal line immediately above it during one field, and with the horizontal line immediately below it during the next field. Even and odd fields are thereby output alternately, each pair of fields forming one frame. After each field, the sensor elements are reset in preparation for integration of the next field. Although the resetting creates noise in the output signal, the reset noise is removed by a signal-processing technique known as correlated double sampling (CDS).
A block diagram of a conventional video camera using a complementary color filter and correlated double sampling is shown in FIG. 1. The camera comprises a lens 1, a single CCD image sensor 2, a first signal-processing circuit 3, a synchronization signal generator or sync generator 4, a CCD driver 5, and a second signal-processing circuit 6. The first signal-processing circuit 3 carries out correlated double sampling, and amplifies the image signal to an appropriate level. The second signal-processing circuit 6 separates the luminance and chrominance components, performs gamma correction and chrominance signal processing, and encodes the results as an output video signal.
This conventional single-CCD color camera is used as follows to obtain a still picture of a subject briefly illuminated by a strobe lamp. Referring to FIG. 2, the synchronization signal generator 4 generates a vertical synchronization signal (A) at regular intervals, and the CCD driver 5 generates CCD readout signals (B1 and B2) at matching intervals. Initially, the subject is not illuminated and the video output signal (D) is black. The strobe lamp is triggered-by a strobe timing signal (C), the lamp turning on for the brief interval while the strobe timing signal is high. Charge accumulates in the CCD pixels during this brief interval. At the next vertical synchronization interval, the accumulated charge is transferred into the vertical transfer registers, first from the pixels in odd-numbered horizontal lines in response to readout signal B1, then from the pixels in the even-numbered horizontal line in response to readout signal B2. The charges from the even lines of pixels mix with the charges from the odd lines of pixels in the vertical transfer registers as described above, producing image information for one video field. The image information is processed by the first signal-processing circuit 3 and second signal-processing circuit 6 to produce a color video image consisting of one field in the output video signal D. After this single field is output, the video signal (D) again becomes black.
Still pictures obtained in this way are used for motion analysis and various other purposes, but their clarity is reduced by their poor vertical resolution. Since only one field is output, only half of the horizontal scanning lines are represented. The attractiveness and usefulness of these pictures would be enhanced if full vertical resolution could be obtained.
It would also convenient if full vertical resolution could be obtained without alteration of the structure of the CCD image sensor or the processing performed by the signal-processing circuits.
An object of the present invention is to provide a video camera that can take still pictures in color with full vertical resolution, equivalent to the pictures taken by an electronic still camera.
A further object is to obtain full-resolution still pictures from a solid-state image sensor and signal processing circuits of the type normally used to generate color moving pictures with interlaced scanning.
The invented video camera uses a solid-state image sensor, having a matrix of sensor elements disposed in even lines and odd lines and a complementary color filter, to generate an interlaced video signal. The video camera also has a driver circuit, an image memory, and a control circuit.
In a still-picture mode, the driver circuit generates a first readout signal and a second readout signal in synchronization with alternate vertical intervals of the interlaced video signal. The first readout signal reads image information from the even lines of sensor elements in the solid-state image sensor. The second readout signal reads image information from the odd lines of sensor elements. The image information read from both even and odd lines is stored separately in the image memory.
The control circuit reads the image information stored in the image memory, combines the image information from the even lines with the image information from the odd lines in one way to generate a first field of the interlaced video signal, and combines the same information in another way to generate a second field of the video signal. The first field and second field constitute a single frame, yielding a still picture with full vertical resolution.